I believe the 56" roller will hold more, than two 44" rollers...The big difference is drawer depth...
The big thing for most people is the smaller overall size of the 56" vs two 44" rollers...
44"-----14,200 cu inches
56"-----28,200 cu inches
72"-----34,300 cu inches
I have a 56" roller cabinet, and it's taller, deeper, and better than the 44"'er...and if you can get it on sale for $ 699.99(I did), it is, in my opinion, a better deal than two 44" cabinets...and, if you ever need more storage, there's a Top Box!!!
If maximum storage and usefulness, is what your after---the 56" Roller Cabinet, Top Box and 5 Drawer Tool Cart will set you back less than $ 1,250.00, if bought on sale...and you'd really be covered...
Or start with a 56" Roller Cabinet, add a 5 Drawer Tool Cart, later, and finish it off with a 56" Top Box, when necessary...
All the side boxes can be bolted on the 56", but I wouldn't do that...I'd just buy a HF 5 drawer Tool cart...
If you could catch the 56" Roller Cabinet, AND the 5 Drawer Tool Cart on sale, you be out about $ 850.00, and you'd have all your storage needs met---until, you bought enough tools to warrant the purchase of the 56" Top Box...
It's your choice, but I love my 56" Roller cabinet, so you know what gets my vote...side by side, with both pushed back against the wall, the 56" Roller Cabinet, is MASSIVE, in comparison to its "little brother"...